1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation which includes means for varying the size and position of a dominant region of the x-ray image.
2. Description of the Prior Art
X-ray diagnostics systems are known which generate x-ray images, the x-ray image frequently including a dominant region of interest within the image, which contains the primary item of medical interest within the image. Such installation include an x-ray image intensifier and a detector for measuring the average image brightness in the dominant region of the output screen of the x-ray image intensifier. Such systems are used for video image reproduction, as well as producing individual images and series of images, with the detector maintaining the average image brightness constant in the dominant region.
It is known to selectively vary, within limits, the size and shape of the dominant region, as described in German OS 31 27 648. In this known installation, a portion of the image is reflected by a mirror out of the parallel light beam path of the optics system, the optics system being disposed between the output luminescent screen of x-ray image intensifier and the video camera. The mirror is disposed directly in the parallel beam path, and the portion of the image reflected out of the beam path by the mirror constitutes the dominant region. Selection of the size and shape of the dominant region is undertaken by a dominant disc which precedes a photomultiplier. The physical size of the dominant disc, however, places limits in the number of different shapes, positions and sizes of the dominant region which can selected, because all of these variations must be accommodated within the dominant disc.
It is generally standard in such x-ray image intensifiers to switch the format of the x-ray image intensifier dependent on the size of the subject under observation and on a desired magnification, so that identical areas in the input luminescent screen illuminate a different area on the output luminescent screen. A magnification of the examination subject can thereby be achieved in a simple manner. A disadvantage in such known systems, however, is that the dominant region changes due to the switching of the x-ray image intensifier. This can be prevented by providing an appropriate opening in the dominant disc for each setting of the x-ray image intensifier, and by correspondingly adjusting the dominant disc at the time a switching of the x-ray image intensifier is undertaken. This technique also requires a large number of openings be provided in the dominant disc, not only for each desired shape and position of the dominant region, but also for each imaging scale of the x-ray image intensifier. Again, this results in a dominant disc which is of an unmanageably large size.